1. Field of the Invention
This invention relates generally to matrix precursors for carbon-carbon composites and specifically to polyarylacetylene (PAA) resins.
2. Description of the Prior Art
Polyarylacetylene (PAA) resin is a superior material as a matrix precursor for carbon/carbon composites. It has a much higher char yield (approximately 95% theoretical and 88% observed char yield) than those attainable in conventional precursors such as phenolic and pitch. The high char-yield of PAA produces carbon/carbon composites with reduced shrinkage and weight loss, and therefore requires fewer reimpregnation cycles to achieve a desired density.
One drawback of PAA as a carbon precursor is the inability to control the crystallinity of the carbon matrix due to its highly cross-linked nature. In order for the cross-linked polymer to rearrange and form a highly graphitic structure, a large number of carbon-to-carbon bonds must be broken and subsequently rearranged. This energy barrier cannot be overcome even upon heat treatments in excess of 2800 degrees Centigrade. As a result, PAA like other thermoset resins, always chars in place to form a poorly ordered glassy or vitreous carbon. Within a composite, PAA displays graphitization localized around the fiber. Although this localized graphitization occurs with heat-treatment to 2400 degrees Centigrade, areas exist in which the matrix remains glassy.
Metals are well known catalysts for graphitization. Nickel is a catalyst for graphitization of PAA, pitch, and phenolic resins but is detrimental to graphite fibers. Boron has also been shown to be an effective graphitizing catalyst for various cokes and glassy-type carbonaceous material. Most catalyst studies were done on thermoplastic carbons for the purpose of understanding the mechanism for catalytic graphitization. According to the prior art, an integration of the catalyst into the polymers was done in an inhomogeneous manner. A powder with a particle size of approximately 1-100 microns is mechanically mixed with the viscous polymer. Segregation of the powder, along with the settling of this second phase, was usually evident.
Therefore, the principal objective of this invention is to improve the use of PAA as a carbon-carbon matrix precursor through control of the PAA microstructure.